Print hammer selection circuit in endless belt line printers

ABSTRACT

Staggered successions of reference character codes that are generated in timed relation to the periodic movement of a typecarrying belt past an aligned row of printing hammers are periodically sampled. The respective samples are routed through selected time slots of successive frames containing information character codes representative of the type characters to be printed. The occurrence of an identity between a shifted sample and the information character code in a particular time slot causes the actuation of an associated one of the printing hammers to print the corresponding type character.

I United States Patent 1 1 3,56 1,354

[72] Inventor Jaroslav Mrkvicka [56] Referen e Cited l N f gg gz UNITEDSTATES PATENTS [2H P 3,066,601 12/1962 Eden 101 93 [22] F1Ied Aug. 12,1968 3,289,576 12/1966 Bloom et a1 101/93 [45] Patented Feb. 9,1971 3303 775 2/1967 101/93 [73] Assignee Vyzkumny ustav matematickych strojulamiuzzl 3,312,174 4/1967 Cunn1ngham.. 101/93 Praha, Czechoslovakla, afi 3 443 514 5/1969 Schwartz 101/93 [32] Priority Aug. 10, 1967 [33]Czechoslovakia Primary ExaminerWil1iam B. Penn [3 1 I PV5,76l-67Attorney-Arthur O. Klein ABSTRACT: Staggered successions of referencecharacter 54 PRINT HAMMER SELECTION CIRCUIT 1N g sf f i a the Q ENDLESSBELT LINE PRINTERS e en 0 a P Ty g e pas an a lgne row 0 pnn 4 Cl 6 D Fmg hammers are periodically sampled. The respective samples films rawmgare routed through selected time slots of successive frames [52] US. Cl101/93, containing information character codes representative of the340/ 172.5 type characters to be printed. The occurrence of an identity[51] Int. Cl B41 j l/20, between a shifted sample and the informationcharacter code B41 j 9/ 14; 606k 15/08 in a particular time slot causesthe actuation of an associated [50] Field of Search I01/93RC, one of theprinting hammers to print the corresponding type 96RC; 340/1461,236(Inquired), 172.5

character.

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JQ SIm'WR QKKA PRINT HAMMER SELECTION CIRCUIT IN ENDLESS BELT LINEPRINTERS BACKGROUND OF THE INVENTION One form of digitally controlledline printer employs an endless belt on which rows of successive typecharacters are suitably embossed. The belt moves periodically past asuccession of aligned printing hammers whose spacing determines thecolumn spacing of the printed line. The hammers are selectively actuatedto press a pressure-sensitive paper tape against the oppositely disposedtype character on the belt and thus provide a corresponding impressionon the tape.

Each hammer is associated with a predetermined time slot in a recurrentframe of incoming data defined by a plurality of information charactercodes. The code occurring in each time slot determines the typecharacter that is to be printed in the column defined by thecorresponding hammer.

Actuation of the printing hammer associated with each time slot occursin response to a detected identity, in such time slot, of the thenoccurring information character code and one of a plurality of referencecharacter codes which are respectively representative of the successivetype characters on the belt. The latter are generated by a suitableselection circuit actuated in synchronism with the periodic movement ofthe successive type characters on the belt past a fixed point.

In the past, selection circuits for moving-belt line printers have beencomplicated, particularly where the spacing between successive ones ofthe type characters on each row of the belt is different from thespacing between successive ones of the printing hammers.

SUMMARY OF THE INVENTION The present invention provides a relativelysimple selection circuit for moving-belt line printers having a typespacing on the belt that is greater than the column spacing of theprinted line by a factor K /Q where '70 is a rational number greaterthan 1 and where P and Q have no common factors. A character generatoroperating in MODULO-P fashion is coupled to the moving belt forgenerating P staggered but otherwise identical successions of thereference character codes corresponding to the successive typedcharacters on the belt. The successions of reference codes aresequentially sampled in successive ones of the frames. A shiftingcircuit routes each sample through an associated set of time slots ofthe corresponding frame; a separate set of time slots is assigned toeach of the staggered successions of reference character codes. Eachidentity between a shifted sample and the simultaneously occurringinformation character code actuates the printing hammer assigned to thetime slot in which the identity occurs.

BRIEF DESCRIPTION OF THE DRAWING The nature of the invention and itsadvantages will appear more fully from the following detaileddescription taken in conjunction with the appended drawing, in which:

FIG. 1 is a block and schematic diagram of a selection circuit for adigitally controlled line printer in accordance with the invention;

FIG. 2 is a set of waveform diagrams illustrating a frame of time slotscoincident with those containing coded information for controlling theline printer of FIG. 1;

FIG. 3 is a pictorial diagram of one form of MODULO-P charactergenerator suitable for use in the selection circuit of FIG. 1;

FIG. 4 is a set of waveform diagrams for the MODULO-3 charactergenerator of FIG. 3;

FIG. 5 is a set of waveform diagrams for a shifting-pulse generatorsuitable for use with the selection circuit of FIG. 1, illustrating thetime relation among the shifting pulses, the frame duration, and thewaveforms of FIG. 4; and

FIG. 6 is a pictorial diagram of an alternative form of charactergenerator for producing the waveforms of FIG. 4.

DETAILED DESCRIPTION Referring to the drawing. FIG. I illustrates adigitally controlled line printer 10 for printing a line of N typecharacters (not shown) on a pressure-sensitive paper web 2A. Theprinting of the line is accomplished with the use of N aligned.solenoid-actuated printing hammers 12-1. 12-2. IZ-N which selectivelypress the web 2 against an endless moving belt 11 on which axiallydisposed rows of typed characters -110 to be printed are embossed. Thebelt 11 is suitably driven by a pair of drive rolls 2IA-21A at constantspeed for translational movement parallel to the hammers 12 toperiodically move each successive type character through positionsdirectly opposite each of the hammers l2.

The solenoids (not shown) of selected hammers 12 are actuated byenergizing associated ones of a plurality of individual power supplies9-1, 9-2, 9-N. The N power supplies 9, in turn, are energized by theapplication thereto of N command bits which are outpulsed over Nchannels 14-1, 14-2, l4-N of a serial-to-parallel converter 8. Apredetermined state of each command bit (eg a binary 1) applied to theassociated one of the power supplies 9 will actuate the hammer 12associated therewith to print the type character on the then-opposedportion of the belt 11.

As is well known in the line printer art, the incoming information whichcontrols the generation of the command bits for each line to be printedmay be contained in successive recurrent frames each having apredetermined number of time slots of duration T. Each frame ofinformation is generated by a suitable external source (represented by acomputer I00) and stored in serial form in a suitable buffer memory 7 ofconventional construction. For example, the buffer 7 may take the formof a delay line having a storage capacity of IT time slots as depictedin FIG. 2, wherein successive frames of the incoming information storedin the buffer may recur at intervals of length Z. Not all of the timeslots in the buffer need be employed for information storage; as shownin FIG. 2, for example, the seventh time slot in each frame may beinactive for information storage purposes and may be used to storesuitable error checking codes if desired.

As indicated below, the identification of an information character codeoccurring in a particular time slot by the line printer selectioncircuit (designated generally at 22A in FIG. 1) will provide the commandbit for the associated hammer 12 to print the required type character.For this purpose the converter 8 is employed to relate the respectivetime slots to separate the ones of the channels 14 for individuallyexciting the associated hammer power supplies 9 in the manner described,e.g., in applicants copending application Ser. No. 729,825, filed May16, I968, and entitled ERROR CHECKING CIRCUIT FOR DIGITALLY CONTROLLEDPRINTERS."

It will be assumed for purposes of the following discussion of theselection circuit 22A that the spacing between successivetype characterson the belt 11 is lktimes the spacing K between the successive hammers12.

The belt 11 is mechanically coupled, as by a link 11A, to a charactergenerator 1. As shown in FIG. 3, the generator I may illustrativelycomprise a rotatable disc 15 which makes one revolution for eachincrement of belt movement equal to the spacing between successive typecharacters thereon. The disc 15 has a first track 151 that cooperateswith a first pickup 161 to provide, during each revolution, an outputpulse having a duration proportional to one-third of the spacing betweensuccessive type characters on the moving belt 11. Each pulse from thepickup 161 is applied to the input of a MODULO 3-counter 16 having threeoutputs in the form of staggered but otherwise identical first pulsetrains at, B and 7 (FIG. 4). Each of the trains (1,3, 'y has a periodT-{proportional to the belt type spacing, and a pulse durationcorresponding to onethird of such spacing.

Referring again to FIG. 3, the first pulse trains 01.13, and whichappear on output lines 101, I02. and 103 of the counter 16, are appliedto separate inputs of a triggcrable character register 17 which providesthree staggered but otherwise identi cal successions Da, DB. and D-y(FIG. 4) of reference character codes in respective. synchronism withthe pulse trains 'y, B, and a. Each of the reference codes in thesuccessions Da, DB, and D7, which are individually outpulsed over lines111, 112, and 113 of the register 17, corresponds to (and is representedin FIG. 4 by a line labeled as) one of the successive type characters onthe belt 11. The construction of such a character register, whichprovides a different code on its output each time it is triggered, iswell known in the art and will not be further discussed here. The basictriggering signals for the register 17 are applied once during eachcycle of revolution of the disc 15 (FIG. 3) by a pickup 171 associatedwith a suitable second track 152 on the disc 15, while the staggeredpulse trains 'y, B, and a are used to operate the register 17 inMODULO-3 fashion similar to that of the counter 16.

Referring again to FIG. 1, the respective successions of referencecharacter codes outpulsed by the generator 1 over the lines 111, 112,and 113 are sampled at the beginning of each frame of information storedon the buffer 7 by means of pulses from a sampling pulse generator 2.(For purposes of the following discussion the frame length Z may betaken to coincide with the pulse length T 2/3 of each of the trains at,B, and y in the manner shown in FIG. 5.) The generator 2 (FIG. 1) may bea MODULO-3 counter of the same general type as the counter 16 of FIG. 3,except that the generator 2 is triggered at time slot intervals T ratherthan at intervals proportional to T 2/3, as was the case with thecounter 16. Thus, the generator 2 provides, on output lines 21, 22, and23, three staggered second pulse trains a, b, and c (FIG. 2) each ofwhich has a pulse length T. The triggering pulses for the generator 2(FIG. 1) are obtained from the computer 100 over a line 20.

The generator 2 also provides a timing pulse over an output line 24 todefine a sampling interval equal to 3T at the beginning of each frame inthe manner shown in FIG. 2.

The reference code successions Da, DB, and D-y on the lines 111, 112,and 113 (FIG. 1) are respectively applied to first inputs 411, 421, and431 of a plurality of sampling gates 41, 42, and 43. The second pulsetrains c, b, and a are respectively applied to second inputs 413, 423,and 433 of the gates 41-43. The timing pulse Q is applied in common tothird inputs 412, 422, and 432 of the gates 41-43. With thisarrangement, the respective character code sequences Da, DB, and U aresampled during successive ones of the first three time slots ofsuccessive frames of the information stored in the buffer 7.

The samples of the sequences Du, DB, and By are applied, via lines 414,424, and 434 to inputs 441, 442, and 443 of an OR gate 44. Thesuccessive samples at the output of the OR gate 44 are applied to aninput 51 of a suitable shift register 5. The register is adapted, in aconventional manner, to store each of the successive samples appliedthereto from the OR gate 44 and thereafter, in response to a successionof pulses S (FIG. 5) applied thereto over an integral number of timeslots, to shift the stored sample through the number of time slotsoccupied by the coincident pulse S.

The timing of the pulses S shown in FIG. 5 is chosen to suecessivelyshift each sample through a set of predetermined time slots uniquelyassigned to the corresponding one of the reference code successions Dot,DB, and Dy. Such timing is effectively obtained with the use of ashifting pulse generator 3. The latter operates, in the manner describedbelow, on the first and second staggered sets of trains at, B, and 'yand a, b, and 0 applied to inputs 34-36 and 31-33, respectively, of thegenerator 3. In particular the pulse sequence S results from theperformance, in the generator 3, of the operation S= a(b+) B(a+) 'y(a+).The generator 3 also provides a succession of gating pulses T. Thepulses T result from the performance, in the generator 3, of theoperation T at: Bh yc and so are complementary to the pulses S,

as clearly shown in FIG. 5. The pulses T are used to selectively open asecond gate 55 (FIG. 1) through a control input 552. During each openperiod of the gate 55, the then-occurring shifted sample in the register5 is transmitted to a first input 61 of a conventional coincidencedecoder 6.

The manner in which the successive samples may be shifted throughpredetermined time slots of successive frames will now be illustrated.It will be noted from FIG. 5 that the first time slot in the firstdepicted frame. i.e., that corresponding to the pulse duration of thefirst train a, manifests the absence of a pulse S and the presence ofapulse T. Thus, in this first time slot, the sample of the reference codesequence Da applied to and stored in the register 5 (FIG. 1) remainsunshifted and is directly applied in the first time slot to thecoincidence decoder 6 through the now-enabled gate 55. Because of thepresence of the pulse S (FIG. 5) in the second and third time slots inthat frame, the next outpulsing of the sample of the sequence Da to thedecoder 6 occurs, during the fourth time slot, through the gate 55 whichis open by virtue of the simultaneous presence of the pulse T. Duringthe fifth and sixth time slots, the pulse S occurs again, so that(ignoring the depicted inactive time slot) the shifted sample isoutpulsed, during the seventh active time slot in the frame, through thegate 55 which is again opened by the pulse T. Thus, a sample of thereference code sequence Da is applied to the input of the coincidencedecoder 6 in synchronism with the pulses T, i.e. during the first,fourth, seventh etc. active time slots of the frame corresponding to thepulse duration of the sequence Da. In a similar manner, it will be notedfrom FIG. 5 that samples of the reference code sequence DB are applied,in synchronism with the pulses T, to the input of the coincidencedecoder 6 during the second, fifth, eighth etc., active time slots ofthe frame corresponding to the pulse duration of the sequence DB.Finally, samples of the reference sequence D-y are applied to the inputof the coincidence decoder during the third, sixth, ninth, etc., activetime slots of the frame corresponding to each pulse duration of thesequence D The printing hammers 12 may be grouped in accordance with thetime slots through which the samples of the successive reference codesequences are routed. In particular, the hammers associated with thefirst, fourth, and seventh columns of the line to be printed arecontrolled by the shifted samples of the sequence Dot. The hammersassociated with the second, fifth, eighth, etc. columns are controlledby the shifted samples of the pulse sequence DB; and the hammerscorresponding to the third, sixth, ninth, etc. columns are controlled bythe shifted samples of the pulse sequence D-y.

The sequence of shifted code samples applied to the input 61 (FIG. 1) ofthe decoder 6 is compared with the information codes in the successiveframes stored in the buffer 7 and applied to a second input 62 of thedecoder 6. Within the decoder 6, comparison is made between the sampleand the coincident information character codes in each of the time slotsassigned to the corresponding one of the code sequences Da, DB and D Asone typical example the shifted samples of the sequence Da are suitablycompared with the contents of the first, fourth, seventh etc., timeslots in the coincident frame of incoming information from the buffer 7.Each coincidence of the shifted sample and the information charactercode in one of the assigned time slots results in the generation of acommand bit which is applied, through the associated channel 14 of theserial-to-parallel converter 8, to the power supply 9 of the associatedprinting hammer 12. The resulting actuation of the hammer solenoidcauses a printout of the type character (on the belt 11) disposedopposite the excited hammer 12. It will be evident that excitation ofthe hammers associated with the code sequences DB and D may beaccomplished in an analogous manner.

An alternative embodiment of the character generator 1 is shown in FIG.6. This embodiment employs a rotatable disc 13 that is encoded, as bysuitable apertures (not shown) thereon,

with the required reference character codes. The disc 13 is mechanicallycoupled. via the link 11A, for rotation once during each movement of thebelt ll through the spacing between successive type characters Threepickups 141, 142, and 143 are associated with respective points of theperiphery of the disc that are spaced at intervals D. The distance Dcorresponds to 2/3 of the type spacing on the belt. In this embodiment,the pickups l4ll43 respectively outpulse both the staggered first trainsa, B and 'y and the correspondingly staggered reference code sequencesDu, DB and D'y. Details of the construction of the disc 13 and thepickups l4l-l43 are well known in the art and will not be discussedfurther here.

in the foregoing, the invention was described in connection with apreferred arrangement thereof. Many other variations and modificationswill now become obvious to those skilled in the art. For example, itwill be appreciated that the ratio of the spacing between successivetype characters on the belt 11 and the successive hammers is notrestricted to the factor 1.5, but may be any rational number P/Q greaterthan one, where P and Q have no common factor. In such a case it isnecessary that the generators l6 and 2, which provide the first andsecond trains at, B, 'y and a, b, c, operate on a MODULOP basis, with Psampling gates being correspondingly required. In such a case thecharacter generator embodiment shown in FIG. 6 would have a spacing Dequal to Q/P times the type spacing of the belt 11. These and othermodifications may be made without departing from the spirit and scope ofthe inventron.

l claim:

1. In a line printer whereina row of N aligned printing hammerscooperate with an endless type carrier that is disposed parallel to therow for movement past the row and wherein the type spacing on thecarrier is greater by an indivisible rational factor P/Q than thespacing between adjacent ones of the hammers, an improved apparatus forselectively actuating the printing hammers in response to codedinformation characters grouped in successive incoming data frames of Nactive time slots each, which comprises:

a reference character generator having P outputs for separatelyproviding P similar sequences of reference character codes individuallycorresponding to the successive types on the carrier, the charactergenerator having P inputs respectively associated with the P outputs,each input being operable when excited for triggering the next referencecode in the sequence on the associated output;

means coupled to the carrier for generating P first pulse trains inMODULO-P fashion at a rate proportional to the spacing of the successivetypes on the carrier;

first means for individually coupling the P first pulse trains to the Pinputs of the character generator to respectively produce the referencecharacter sequences in synchronism therewith;

a coincidence decoder having first and second inputs;

means synchronized with the occurrence of the successive data frames ofcoded information characters for cyclically gating, to the first inputof the decoder, samples of each of the outputs of the charactergenerator at intervals of P time slots and for a duration of one timeslot, whereby the time slots occupied by the samples from each output ofthe character generator are mutually distinct;

means for applying the successive frames of coded information charactersto the second input of the decoder;

a serial-to-parallel converter coupled to the output of the decoder andhaving N outputs corresponding to the N time slots of each data frame;and

second means for individually coupling the outputs of the converter tothe printing hammers.

2. Apparatus as defined in claim 1, in which the generating meanscomprises, in combination, a disc rotatably coupled to the type carrier,the disc having first and second signal tracks; a first pickup devicecoupled to the first track for producing, during each revolution of thedisc, a pulse having a duration corresponding to of the type spacing onthe type carrier; a

MODULO-P counter coupled to the output of the first pickup device; and asecond pickup device coupled to the second track for generating animpulse once during each revolution of the disc; and in which the firstcoupling means comprises means for transmitting the outputs of theMODULO-P counter and the second pickup device to the charactergenerator.

3. Apparatus as defined in claim 1, in which the generating meanscomprises, in combination; a coded disc rotatably coupled to the typecarrier, and at least a pair of pickup devices coupled to the peripheryof the disc, the angular spacing between the pickups corresponding toQ/P times the distance between adjacent types on the type carrier.

4. In a line printer wherein a row of N aligned printing hammerscooperate with an endless type carrier that is disposed parallel to therow for movement past the row and wherein the type spacing on thecarrier is greater by an indivisible rational factor P/Q than thespacing between adjacent ones of the hammers, an improved apparatus forselectively actuating the printing hammers in response to codedinformation characters grouped in successive incoming data frames of Nactive time slots each, which comprises:

a reference character generator having P outputs for separatelyproviding similar sequences of reference character codes individuallycorresponding to the successive types on the carrier, the charactergenerator having P inputs respectively asspCiated with the P outputs,each input being operable when excited for triggering the next referencecode in the sequence on the associated output;

first means coupled to the carrier for generating P first pulse trainsin MODULO-P fashion at a rate proportional to the spacing of thesuccessive types on the carrier;

means for individually coupling the P first pulse trains to the P inputsof the character generator to respectively produce the referencecharacter sequences in synchronism therewith;

second means operative in synchronism with each time slot in a dataframe for generating P second pulse trains in MODULO-P fashion, eachsecond pulse train having a pulse length equal to that of a time slot;

normally disabled first gates each having first and second inputs;

means for individually coupling the P outputs of the character generatorto the first inputs of the first gates;

means for individually applying the P second pulse trains to the secondinputs of the first gates;

means for opening the first gates during each frame for P time slots;

an OR gate coupled to the outputs of the first gate;

storage means including a shift register coupled to the output of the ORgate and having a control input, the presence of a pulse in a given timeslot at the control input causing the register to shift to the nextsucceeding time slot;

a normally disabled second gate coupled to the output of the storagemeans and having a control input;

3, a coincidence decoder having first and second inputs; 5 2 means forcoupling the output of the second gate to the first input of thedecoder;

means responsive to the first and second pulse trains for generatingthird and fourth complementary pulse trains, the amplitude of the thirdpulse train being the sum of the products of the amplitudes ofcorresponding ones of the first and second pulse trains;

means for applying the fourth pulse train to the control input of thestorage means;

means for applying the third pulse train to the control input of thesecond gate;

means for applying each data frame to the second input of thecoincidence decoder;

a serial-to-parallel converter coupled to the output of the decoder; and

means individually coupling the outputs of the converter to the printinghammers.

1. In a line printer wherein a row of N aligned printing hammers cooperate with an endless type carrier that is disposed parallel to the row for movement past the row and wherein the type spacing on the carrier is greater by an indivisible rational factor P/Q than the spacing between adjacent ones of the hammers, an improved apparatus for selectively actuating the printing hammers in response to coded information characters grouped in successive incoming data frames of N active time slots each, which comprises: a reference character generator having P outputs for separately providing P similar sequences of reference character codes individually corresponding to the successive types on the carrier, the character generator having P inputs respectively associated with the P outputs, each input being operable when excited for triggering the next reference code in the sequence on the associated output; means coupled to the carrier for generating P first pulse trains in MODULO-P fashion at a rate proportional to the spacing of the successive types on the carrier; first means for individually coupling the P first pulse trains to the P inputs of the character generator to respectively produce the reference character sequences in synchronism therewith; a coincidence decoder having first and second inputs; means synchronized with the occurrence of the successive data frames of coded information characters for cyclically gating, to the first input of the decoder, samples of each of the outputs of the character generator at intervals of P time slots and for a duration of one time slot, whereby the time slots occupied by the samples from each output of the character generator are mutually distinct; means for applying the successive frames of coded information characters to the second input of the decoder; a serial-to-parallel converter coupled to the output of the decoder and having N outputs corresponding to the N time slots of each data frame; and second means for individually coupling the outputs of the converter to the printing hammers.
 2. Apparatus as defined in claim 1, in which the generating means comprises, in combination, a disc rotatably coupled to the type carrier, the disc having first and second signal tracks; a first pickup device coupled to the first track for producing, during each revolution of the disc, a pulse having a duration corresponding to P of the type spacing on the type carrier; a MODULO-P counter coupled to the output of the first pickup device; and a second pickup device coupled to the second track for generating an impulse once during each revolution of the disc; and in which the first coupling means comprises means for transmitting the outputs of the MODULO-P counter and the second pickup device to the character generator.
 3. Apparatus as defined in claim 1, in which the generating means comprises, in combination, a coded disc rotatably coupled to the type carrier, and at least a pair of pickup devices coupled to the periphery of the disc, the angular spacing between the pickups corresponding to Q/P times the distance between adjacent types on the type carrier.
 4. In a line printer wherein a row of N aligned printing hammers cooperate with an endless type carrier that is disposed parallel to the row for movement past the row and wherein the type spacing on the carrier is greater by an indivisible rational factor P/Q than the spacing between adjacent ones of the hammers, an improved apparatus for selectively actuating the printing hammers in response to coded information characters grouped in successive incoming data frames of N active time slots each, which comprises: a reference character generator having P outputs for separately providing similar sequences of reference character codes individually corresponding to the successive types on the carrier, the character generator having P inputs respectively associated with the P outputs, each input being operable when excited for triggering the next reference code in the sequence on the associated output; first means coupled to the carrier for generating P first pulse trains in MODULO-P fashion at a rate proportional to the spacing of the successive types on the carrier; means for individually coupling the P first pulse trains to the P inputs of the character generator to respectively produce the reference character sequences in synchronism therewith; second means operative in synchronism with each time slot in a data frame for generating P second pulse trains in MODULO-P fashion, each second pulse train having a pulse length equal to that of a time slot; normally disabled first gates each having first and second inputs; means for individually coupling the P outputs of the character generator to the first inputs of the first gates; means for individually applying the P second pulse trains to the second inputs of the first gates; means for opening the first gates during each frame for P time slots; an OR gate coupled to the outputs of the first gate; storage means including a shift register coupled to the output of the OR gate and having a control input, the presence of a pulse in a given time slot at the control input causing the register to shift to the next succeeding time slot; a normally disabled second gate coupled to the output of the storage means and having a control input; a coincidence decoder having first and second inputs; means for coupling the output of the second gate to the first input of the decoder; means responsive to the first and second pulse trains for generating third and fourth complementary pulse trains, the amplitude of the third pulse train being the sum of the products of the amplitudes of corresponding ones of the first and second pulse trains; means for applying the fourth pulse train to the control input of the storage means; means for applying the third pulse train to the control input of the second gate; means for applying each data frame to the second input of the coincidence decoder; a serial-to-parallel converter coupled to the output of the decoder; and means individually coupling the outputs of the converter to the printing hammers. 